Problem: Solve for $k$ : $4 = k - 9$
Answer: Add $9$ to both sides: $ \begin{eqnarray} \\ \hphantom{4 {+ 9}} &\hphantom{=}& \hphantom{k} \\ 4 &=& k - 9 \\ \\ {+9} && {+9} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ 4 {+ 9} &=& k \\ \end{eqnarray} $ Simplify. $ k = 13$